1. Field of the Invention
The present invention relates to a method for manufacturing a radiation image conversion panel of a stimulable phosphor type.
2. Description of the Related Art
A radiation image recording and reproducing method using a stimulable phosphor is known, which can replace conventional radiography. This method utilizes a radiation image conversion panel containing a stimulable phosphor (that is, an accumulative phosphor sheet). In this method, radiation transmitted through a subject or emitted from an object to be examined is absorbed into the stimulable phosphor of the above-mentioned panel, and the stimulable phosphor is excited by an electromagnetic wave (excitation light) such as visible light or infrared light in a time sequential manner so as to allow radiation energy accumulated in the stimulable phosphor to emit as fluorescent emission (stimulable emission). Subsequently, an electric signal is obtained by photoelectrically reading the fluorescence, and based on the obtained electric signal, a radiation image of the subject or object to be examined is reproduced as a visible image. The panel for which the reading has been completed is made ready for subsequent photographing after remaining images have been deleted therefrom. That is, the radiation image conversion panel can be used repeatedly.
The above-described radiation image recording and reproducing method has an advantage in that a radiation image having much information can be obtained using a radiation dosage much smaller than in conventional radiography using a combination of a radiographic film and intensifying screen. Further, a radiographic film is consumed each time an image is produced in conventional radiography. In contrast, a radiation image conversion panel in the radiation image recording and reproducing method is advantageous from the standpoint of resource protection and economic efficiency due to reusability.
The stimulable phosphor is a phosphor which exhibits stimulable emission when excitation light is irradiated thereon after having being irradiated with radiation. For practical use, a phosphor which emits stimulable emission in a wavelength range from 300 to 500 nm by excitation light in a wavelength range from 400 to 900 nm is generally employed. An example of the stimulable phosphor conventionally used in radiation image conversion panels, is a rare earth-activated alkaline earth metal halide based phosphor.
The radiation image conversion panel has a basic structure comprised of a support and a stimulable phosphor layer (hereinafter referred to merely as “phosphor layer”) provided on the support surface. However, a support is not necessarily needed if the phosphor layer is self-supporting. The phosphor layer is usually comprised of a stimulable phosphor and a binder which contains and supports the phosphor in a state of dispersion. However, phosphor layers comprised only of an aggregate of stimulable phosphors formed by vapor deposition or sintering and containing no binder are also known. Further, radiation image conversion panels having a phosphor layer in which high polymer materials are impregnated in voids in an aggregate of stimulable phosphors are also known. All of the above-described phosphor layers exhibit the property of stimulable emission when irradiated with excitation light after having absorbed radiation such as X-rays. Accordingly, the radiation transmitted through a subject or emitted from an object to be examined is absorbed in the phosphor layer of the radiation image conversion panel in proportion to the amount of the radiation, and a radiation image of the subject or the object to be examined is formed as an image of accumulated radiation energy in the radiation image conversion panel. The accumulation image can be released as stimulable emission light by irradiation with excitation light, and photoelectrically reading the stimulable emission light and converting the same into an electric signal, so that the radiation energy accumulation image can be made into a visible image.
The above-described rare earth-activated alkaline earth metal halide based stimulable phosphor is excellent in sensitivity, and when this stimulable phosphor is used in a radiation image conversion panel, it produces a radiation reproduction image having high sharpness and is thereby provided as a stimulable phosphor improved for practical usages. However, with the advance of a radiation image recording and reproducing method being put into practical use, there has been increased demand for stimulable phosphors having further improved characteristics.
The phosphor layer is generally formed in such a manner that a phosphor layer coating liquid prepared by adding a stimulable phosphor and a binder to a suitable solvent and then dispersing and dissolving the materials in the solvent to form a phosphor layer coating liquid. Thereafter, the coating liquid is applied and dried on the surface of a support. However, if the phosphor layer coating liquid produces a phosphor layer having coarse grains, graininess deteriorates in the resulting radiation image conversion panel.
In order to remove coarse grains, there is disclosed a method in which an aggregate reduction process is performed by again dispersing a calcined stimulable phosphor in a dispersion medium, resulting in a slurry, which is subjected to wet classification (Japanese Patent Application Laid-Open (JP-A) No. 11-106748). According to this method, coarse grains formed in calcining the stimulable phosphor can be substantially reduced and a radiation image conversion panel having improved graininess can be obtained.
However, the slurry, which has been subjected to wet classification, is dried and grains of the stimulable phosphor thereby form again. Thereafter, the stimulable phosphor is used for preparing the phosphor layer coating liquid. In drying the coating liquid, there is the possibility that the grains will again aggregate into coarse grains to at least some degree.